Hydrous stannic oxide (HSO) was synthesized in the laboratory and its systematic Cr (VI) adsorption behaviour was studied by means of batch experiments. The particle size of HSO used was in the range of 140 to 290µm. The variable parameters viz. the effects of pH, concentration of Cr (VI) and time of contact etc. are here reported. The optimum pH and time of contact required for maximum adsorption was found to be 2.0 and nearly 90 min, respectively. The experimental equilibrium adsorption data are tested for the Langmuir, Freundlich, Temkin and Redlich-Peterson equations. Results indicate the following order to fit the isotherms equations: Redlich-Peterson > Temkin > Freundlich > Langmuir. Different kinetic models have been applied to fit the experimental kinetic data. The results are compared, and indicated that the best fit is obtained with the Lagergren or pseudo first-order and the power-function models. A discussion on the adsorption mechanism with respect to the thermodynamic parameters leads to two possible interpretations: One is the exothermic nature of the adsorption process and the other is the ion-ion type electrostatic interaction between the adsorbent and adsorbate ion.
Synthetic crystalline hydrous ferric oxide (CHFO) (particle size 0.14 to 0.29 mm) has been used systematically for adsorptive chromium(VI) removal from contaminated water. Batch experiments were performed as a function of pH, contact time, solute concentration, and regeneration of adsorbents. Column experiments were performed for breakthrough points in the presence and absence of other ions and treatment of industrial effluent. The optimum pH range was 2.0 to 4.0. The adsorption kinetic data could be described well by both second-order and pseudo-firstorder models. The isotherm adsorption data at 30 6 28C obeyed the Langmuir model best. The monolayer adsorption capacity was 35.7 mg/g. Chromium(VI)-rich CHFO could be regenerated up to 89 6 1% with 2.0 M sodium hydroxide. Regenerated column reuse showed a decrease (10 to 12%) in breakthrough capacity. Finally, the CHFO-(dried at 3008C) packed column was used for the recovery (98.5 6 1.0%) of chromium(VI) from contaminated industrial waste effluent of Hindustan Motor Limited (Hooghly, West Bengal, India). Water Environ. Res., 78, 986 (2006).
This paper deals with the various sources of error in different steps of the procedure for determination of sulfur trioxide in gypsum samples, using hydrochloric acid as solvent for gypsum and barium chloride to precipitate barium sulfate. The paper suggests reduction in the amount of acid and barium chloride used in order to minimize errors. Thus, it justifies the need for the revision of Section 12 of ASTM, Chemical Analysis of Gypsum and Gypsum Products (C 471). ASTM C 471 recommends use of 50 mL of 1 + 5 hydrochloric acid (sp gr 1.19) as solvent for a 0.5-g gypsum sample and 20 mL of 10% barium chloride as precipitant. This results in too high a concentration of acid and chloride ions in reaction solution before and after precipitation, justifiable only for dissolution of impure gypsum samples and formation of coarse barium sulfate crystals to give rapid filtering. The method is silent about the final volume to be made of reaction solution, which determines the acidity before precipitation. It is also silent about the use of and the type of filtering crucibles. The greatest source of error is the occlusion of chloride ions caused by both hydrochloric acid and barium chloride in addition to adsorption of H+, coprecipitation of barium chloride and Ca++ ions, and increased solubility of barium sulfate in acidic solution. Therefore, the following suggestions have been made (1) reduce the initial amount of hydrochloric acid used as solvent to 25 mL of 1 + 5 hydrochloric acid for the 0.5-g test sample, (2) use 20 mL of 6% barium chloride solution (preferably old), (3) make the volume of the reaction solution up to 400 to 500 mL before precipitation of barium sulfate, and (4) use filtering crucibles. These changes will increase the accuracy of the sulfate determination by minimizing error caused by high acid and chloride concentration and by maintaining quick dissolution and filtering. Analyses performed with these changes have been found to be consistent and accurate.
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